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Chapter 18
Patterns of Chromosome Inheritance
Points to Ponder• What is the structure of chromosomes?• What is the cell cycle and what occurs during each of its stages?• Explain what mitosis is used for, in what cells and the 4 stages.• Explain the 2 divisions of meiosis.• What is meiosis used for and in what cells?• Compare and contrast mitosis and meiosis.• Compare and contrast spermatogenesis and oogenesis.• What are trisomy and monosomy?• What most often causes these changes in chromosome
number?• What are the syndromes associated with changes in sex
chromosomes?• Explain the 4 changes in chromosome structure.
Chromosomes: A review
• Humans have 46 chromosomes that are in 23 pairs within a cell’s nucleus– Pairs of chromosomes are called homologous chromosomes– Autosomes are the 22 pairs of chromosomes that control traits
that do not relate to gender of an individual– Sex chromosomes are the 1 pair that contain the genes that do
control gender
• Cells (body cells) that have 46 (2N) chromosomes are called diploid
• Cells (sex cells) that have only 23 (N) chromosomes not in pairs are called haploid
18.1 Chromosomes and cell cycle
What is a karyotype?18.1 Chromosomes and cell cycle
The cell cycle• 2 parts:
1. Interphase:• G1 stage – cell doubles its organelles; cell grows
in size• S stage – DNA replication occurs
• G2 stage – proteins needed for division are synthesized
2. Cell division (mitosis and cytokinesis):• Mitosis – nuclear division • Cytokinesis – cytoplasmic division
18.1 Chromosomes and cell cycle
The cell cycle
18.1 Chromosomes and cell cycle
Chromosome structure in mitosis
• Chromosomes contain both DNA and proteins (collectively called chromatin)
• Chromosomes that are dividing are made up of 2 identical parts called sister chromatids
• The sister chromatids are held together at a region called the centromere
18.2 Mitosis
The spindle in mitosis
• Centrosome – the microtubule organizing center of the cell
• Aster – an array of microtubules at the poles (ends of the cell)
• Centrioles – short cylinders of microtubules that assist in the formation of spindle fibers
18.2 Mitosis
Overview of mitosis
• A diploid cell makes and divides an exact copy of its nucleus
• Used in cell growth and cell repair
• Occurs in body cells• 4 phases:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
18.2 Mitosis
1. Mitosis: Prophase
• Chromosomes condense and become visible
• Nuclear envelope fragments
• Nucleolus disappears
• Centrosomes move to opposite poles
• Spindle fibers appear and attach to the centromere
18.2 Mitosis
2. Mitosis: Metaphase
• Chromosomes line up at the middle of the cell (equator)
• Fully formed spindle
18.2 Mitosis
3. Mitosis: Anaphase
• Sister chromatids separate at the centromeres and move towards the poles
18.2 Mitosis
4. Mitosis:Telophase and cytokinesis
• Chromosomes arrive at the poles
• Chromosomes become indistinct chromatin again
• Nucleoli reappear• Spindle disappears• Nuclear envelope
reassembles• Two daughter cells are
formed by a ring of actin filaments (cleavage furrow)
18.2 Mitosis
Overview of meiosis
• Two nuclear divisions occur to make 4 haploid cells
• Used to make gametes (egg and sperm)
• Occurs in sex cells
• Has 8 phases (4 in each meiosis I & II)
18.3 Meiosis
Meiosis I• Prophase I
– Homologous chromosomes pair (synapsis) and crossing-over occurs in which there is exchange of genetic information
• Metaphase I– Homologous pairs lined at the equator
• Anaphase I– Homologous chromosomes separate and move towards
opposite poles
• Telophase I– 2 daughter cells result each with 23 duplicated chromosomes
18.3 Meiosis
What is crossing over?
• Crossing-over is the exchange of genetic information between nonhomologous sister chromatids during synapsis
• This occurs during prophase I of meiosisand increases genetic variation
18.3 Meiosis
Meiosis I18.3 Meiosis
Meiosis II• Prophase II
– Chromosomes condense again
• Metaphase II– Chromosomes align at the equator
• Anaphase II– Sister chromatids separate to opposite poles
• Telophase II– 4 daughter cells result each with 23 unduplicated chromosomes
18.3 Meiosis
Meiosis II18.3 Meiosis
Mitosis vs. Meiosis
• Growth and repair of cells
• Occurs in body cells
• 1 division
• Results in 2 diploid genetically identical cells
• Formation of gametes
• Occurs in sex cells
• 2 divisions
• Results in 4 haploid genetically different cells
18.4 Comparison of meiosis and mitosis
Comparing mitosis and meiosis
18.4 Comparison of meiosis and mitosis
Comparing mitosis and meiosis
18.4 Comparison of meiosis and mitosis
The production of sperm and eggs
• Spermatogenesis– Process of making sperm
in males– A continual process after
puberty – About 400 million sperm
are produced per day
• Oogenesis– Process of making eggs in
females– During meiosis 1 egg and 3
polar bodies are formed– Polar bodies act to hold
discarded chromosomes and thus disintegrate
– Normally 1 egg per month is produced and ~500 during the entire reproductive cycle
18.4 Comparison of meiosis and mitosis
The production of sperm and eggs18.4 Comparison of meiosis and mitosis
Changes in chromosome number
• Nondisjunction occurs when both members of a homologous pair go into the same daughter cell during meiosis I or when sister chromatids fails to separate in meiosis II.
• Results of nondisjunction:– Monosomy: cell has only 1 copy of a chromosome
e.g. Turner syndrome (only one X chromosome)
– Trisomy: cell has 3 copies of a chromosomee.g. Down syndrome (3 copies of chromosome 21)
18.5 Chromosome inheritance
Changes in chromosome number
18.5 Chromosome inheritance
Changes in sex chromosome number
• Turner syndrome (X) – short stature, broad shouldered with folds of skin on the neck, underdeveloped sex organs, no breasts
• Klinefelter syndrome (XXY) – underdeveloped sex organs, breast development, large hands and long arms and legs
• Poly-X female (XXX, XXXX) – XXX tends to be tall and thin but not usually retarded– XXXX are severely retarded
• Jacobs syndrome (XYY) – tall, persistent acne, speech and reading problems
18.5 Chromosome inheritance
Changes in sex chromosome number18.5 Chromosome inheritance
Changes in chromosome structure
• Deletions – loss of a piece of the chromosome (e.g. Williams syndrome)
• Translocations – movement of chromosome segments from one chromosome to another nonhomologous chromosome (Alagille syndrome)
• Duplications – presence of a chromosome segment more than once in the same chromosome
• Inversions – a segment of a chromosome is inverted 180 degrees
18.5 Chromosome inheritance
Changes in chromosome structure18.5 Chromosome inheritance
Changes in chromosome structure18.5 Chromosome inheritance
